ISOPODA OP THE 'LIGHTNING' AND OTHEK EXPEDITIONS. 79 



the last three forwards. Their minute microscopic structure is very varied, since they 

 are furnished with setae and spines of every kind of complex structure, so that they will 

 be found in their minute armature to afford very reliable specific characters. 



Uropods setaceous, consisting of a basal joint and one or two filamentary branches 

 which vary greatly in length. 



Eespiration by means of a branchial chamber, which is situated beneath the sides of 

 the hinder portion of the carapace. Heart occupying the earlier segments of the 

 perseon. Eggs carried in a pouch beneath the body, which is formed of thin plates, 

 which may either spring from the fourth free segment alone, or consist of four pairs 

 attached to four segments. 



Synopsis of the Families of Chelifera. 

 Upper antennae ■with two flagella. Second maxillEe well developed and setose. 



Secoud gnatliopods -witli a large broad flat hand, suited for burrowi.ig .... Apseudid;k. 

 Upper antennae with a single flagellum. Second maxillse rudimentary and naked. 



Second gnathopods -with narrowed joints, and fitted for walking Tanaid-b. 



Family I. APSEUDID^. 



Animal narrow, produced, depressed, the carapace usually laterally keeled. Segments 

 of pleon well defined, narrower than those of perseon. Rostrum usually well developed, 

 rarely absent. Ocular lobes commonly somewhat pyriform, occasionally taking the 

 form of strong spine-processes. 



Upper antennse situated at the anterior corners of the carapace, with two multiarti- 

 culate flagella. Lower antennae with their bases close together and appressed, lying 

 between and below the upper pair ; the second joint of the peduncle is often furnished 

 with an articulated oblong or linear antennal scale, which is ciliated all round ; flagellum 

 multiarticulate. 



Mandibles well developed, with a three-jointed palp. First maxillae consisting of 

 two members, and furnished with a backward-directed two-jointed palp, which termi- 

 nates in two setae. Second maxillae three-lobed, the lobes setose and spinose. Maxilli- 

 peds with a four-jointed palp and large branchial lamina. 



The flrst gnathopods are strong and chelate, the inner margin of the finger and thumb 

 usually tuberculated in the male and serrate in the female. 



Second gnathopods with the three last joints and especially the hand flattened ; hand 

 oblong or subovate, surrounded with numerous flattened spines, the number, character, 

 and arrangement of which afi'ord good specific characters ; finger very strong, usually 

 toothed ; the whole limb is powerfully built and well adapted for digging. Both pairs 

 of gnathopods are usually furnished with a minute, two-jointed palp attached to the 

 coxa — the rudimentary representatives of the often largely developed palps (exopodites) 

 of the Podophthalmous Crustacea. 



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